It is known to produce polymer dispersions in the presence of starch or starch derivatives acting as a protective colloid. In the emulsion polymerization process described in Published Japanese patent application no. 58/87, 105, 0.5% to 5% by weight graftable, ethylenically unsaturated monomers are polymerized together with other monomers and 10% to 50% by weight of a polysaccharide having a low molecular weight in an aqueous solution. The monomers may be mono- or diolefinic compounds and will form stable lactices only in the critical proportions which have been stated.
Paper sizing agents described in Published European patent application no. 257,412 are based on copolymer dispersions prepared by emulsion polymerization of acrylonitrile, acrylic esters and optionally other ethylenically unsaturated compounds, such as unsaturated carboxylic acids, in the presence of broken down starch. Copolymer dispersions having a relatively low solids content of about 20% to 30% by weight are apparent from the examples.
Published European patent application no. 134,449 discloses a process of producing an aqueous polymer dispersion by the emulsion polymerization of ethylenically unsaturated compounds in the presence of starch and at least 0.6% by weight starch derivatives. Before the polymerization begins, 0.5% by weight starch derivatives acting as a protective colloid are added to the reaction mixture. In this process, organic hydroperoxides are used as radical-forming initiators at reaction temperatures of 10.degree. C. to 100.degree. C. The resulting polymer dispersions have relatively large particle diameters of 235 nm to 3500 nm. The polymer dispersions produced by the aforesaid process may be used, e.g., as adjuvants in the production of textiles and paper.
Such known processes and the polymer dispersions obtained therefrom are not yet satisfactory in all cases in industrial practice as the solids content of such dispersions generally are uneconomically low or said dispersions have high viscosities which result in disadvantages during processing.
It is therefore an object of the present invention to provide a process for the production of polymer dispersions with the aid of starch which can be carried out in a simple manner and which result in stable, flowable, aqueous polymer dispersions having a low viscosity and a high solids content of at least 50% by weight. It is a further object of this invention to provide such a process for the production of polymer dispersions, wherein the polymer particles have a small particle diameter.
Additional objects of this invention will become apparent from the following discussion.